The use of viruses to define the synaptic organization of neuronal circuitry has experienced an explosive growth over the past decade. This experimental approach is the most widely used method to provide a polysynaptic perspective on the functional architecture of the nervous system. Consequently, it has proven to be increasingly popular among neuroscientists whose goal is to define ensemble organization of populations of neurons devoted to specific functions. The goal of this application is to establish a state of the art National Resource Center that will a) serve as a technical and intellectual resource for those interested in using viral transneuronal tracing, b) develop improved transneuronal tracing technologies and make them available to investigators throughout the United States via access to center resources and training, c) serve as a repository for well-characterized reagents essential to the application of the method, and d) stimulate collaborative multidisciplinary studies of mechanisms underlying viral neuroinvasiveness and pathogenesis. Our efforts to establish this National Resource Center are based upon five fundamental tenets. First, there is a general consensus that the method offers a powerful means for functional dissection of neural circuitry, but implementation of the procedures is a costly endeavor beyond the reach of most investigators. Second, recent literature clearly demonstrates that there are valuable and important avenues that can be pursued to improve this technology. A Center would provide the necessary resources and focus to energize this technology development and make it more widely available. Third, there is historical precedent demonstrating that multidisciplinary research in this area rapidly advances the understanding of fundamental principles in multiple fields. A Center would provide the focus and platform for developing such interactions. Fourth, the University of Pittsburgh is the home for a surprising number of investigators who have been influential in the development and application of this methodology and also have productive collaborative relations with other prominent investigators who have been similarly influential. Thus, the Center would be established and successful within a relatively short period of time. Fifth, special facilities already exist at the University of Pittsburgh that meet the unique requirements for the use of alpha herpesviruses and rabies virus in multiple species, including non-human primates. Collectively, these observations support the conclusion that establishment of a Center at the University of Pittsburgh will provide a unique National Resource that will contribute substantially to efforts to improve our understanding of nervous system function and organization.